1. Field of the Invention
The present invention relates to a polarizer by which light leakage based on changes of polarizing element axes caused by a change of a viewing angle between polarizing elements disposed in the form of crossed-Nicol can be prevented in a wide range of the visible light to there by achieve liquid-crystal display of a wide viewing angle, or the like.
The present application is based on Japanese Patent Application No. 2000-340416, which is incorporated herein by reference.
2. Description of the Related Art
In polarizing elements disposed in the form of crossed-Nicol, there was a problem that light leakage might occur when the azimuth was changed to an oblique one even in the case where the light could be cut off normally in a normal-line (frontal) direction. This was because the relationship in crossed-Nicol optical axis is displaced or collapsed between the polarizing elements due to the change of the apparent angle caused by the oblique view. As a background-art device to solve the light leakage problem caused by such an azimuth angle, there was known a polarizer in which a transparent protective film exhibiting birefringence with a retardation of from 190 to 320 nm and with Nz (which will be described later) of from 0.1 to 0.9 was disposed so that the slow axis of the transparent protective film is parallel with the absorption axis of a polarizing element (see Unexamined Japanese Patent Publication No. Hei. 4-305602).
The background-art polarizer was provided to compensate for the displacement in absorption axis or the like between polarizing elements due to the change of the viewing angle as follows. As a transparent protective film to be bonded to one or each of opposite surfaces of a polarizing element for improving durability against penetration of moisture or the like, a film exhibiting retardation characteristic of about a half wavelength with respect to visible light was used instead of an isotropic transparent protective film constituted by a triacetylcellulose (TAC) film or the like exhibiting little birefringence which had been generated otherwise. There was, however, a problem that the compensating measure could not cope with wavelength dispersion.
That is, wavelength dispersion, which is a phenomenon that the retardation varies in accordance with the wavelength, generally occurs in a retardation film. Hence, the function of the retardation film as a half-wave plate works only for light with a specific wavelength. For light with the other wavelengths, the retardation film cannot function as a half-wave plate accurately, so that the light with the other wavelengths is inferior in the characteristic of linear polarization. There therefore arises a coloring problem. Incidentally, when the characteristic of the retardation film is optimized to compensate for light with a wavelength near to 550 nm exhibiting a maximum value in luminous efficiency, light with the other wavelengths is colored in blue because the condition for the light is displaced from the aforementioned optimizing condition. Hence, when the retardation film is applied to a liquid-crystal display device or the like, the coloring problem reveals itself as a problem in deterioration of neutral characteristic of display.